Conventionally, the improvement of a transmission capacity of a subscriber network is required due to the Internet and multimedia communication traffics along with the advancement of information technology (IT). As one of technologies of improving the transmission capacity of a subscriber network, a wavelength division multiplexing passive optical network (WDM-PON) comes into the spotlight.
FIG. 1 is a view illustrating the configuration of the entire network of a conventional wavelength division multiplexing passive optical network (WDM-PON) according to the prior art, and FIG. 2 is a view illustrating the optical transmittance characteristics of an optical demultiplexer (ODMX) of an optical line terminal (OLT) and an optical demultiplexer (ODMX) of an remote node (RN) according to the prior art.
As shown in FIG. 1, the conventional wavelength division multiplexing passive optical network (WDM-PON) includes an optical line terminal (OLT) 100, a remote node (RN) 150, and a plurality of optical network units (ONUs) 160a, 160b, . . . , 160n. 
The optical line terminal (OLT) 100 includes N channel cards. In FIG. 1, for the sake of convenience of explanation, there is described the case where the optical line terminal 100 includes six channel cards and the number of the optical network units (ONUs) are six. That is, the optical line terminal (OLT) 100 includes first to sixth channel cards 111a to 111n. In addition, the optical line terminal (OLT) 100 includes a first optical demultiplexer (ODMX) 112.
The remote node (RN) 150 is connected to the optical line terminal (OLT) 100 through a single optical line 130, and includes a second optical demultiplexer (ODMX) 151. First to sixth optical network units (ONUs) 160a to 160n are respectively connected to the second optical demultiplexer (ODMX) 151 of the remote node (RN) 150 through the single optical line 130.
Since the wavelength division multiplexing passive optical network (WDM-PON) employs different optical wavelength according to the respective channels, it can be implemented based on a principle that although the same optical line is used, respective optical signals are not influenced by each other. In FIG. 1, if it is assumed that the optical transmittance characteristics of the first optical demultiplexer (ODMX) 112 of the optical line terminal (OLT) 100 and the second optical demultiplexer (ODMX) 151 of the remote node (RN) 150 is the same as that shown in FIG. 2, in the case where the wavelengths of downward optical signals that are respectively transmitted to the first to sixth optical network units (ONUs) 160a to 160n from the optical line terminal (OLT) 100 are λ1d to λ6d, and the wavelengths of upward optical signals that are respectively transmitted to the optical line terminal (OLT) 100 from the first to sixth optical network units (ONUs) 160a to 160n are λ1u to λ6u, the wavelength division multiplexing passive optical network (WDM-PON) is implemented based on a principle that six independent communication channels are formed between the optical line terminal (OLT) 100 and the first to sixth optical network units (ONUs) 160a to 160n. 
In order for the wavelength division multiplexing passive optical network (WDM-PON) to be operated normally, it is important that the wavelengths λ1d to λ6d of the downward optical signals that are respectively transmitted to the first to sixth optical network units (ONUs) 160a to 160n from the optical line terminal (OLT) 100, and the wavelengths λ1u to λ6u of the upward optical signals that are respectively transmitted to the optical line terminal (OLT) 100 from the first to sixth optical network units (ONUs) 160a to 160n be identical to or be not deviated by a certain level from the center wavelengths for the respective channels in the optical transmittance characteristics of the first optical demultiplexer (ODMX) 112 and the second optical demultiplexer (ODMX) 151, which are shown in FIG. 2. That is, it is indispensable in which the wavelengths of the downward optical signal of the optical line terminal (OLT) 100 and the wavelengths of the upward optical signals of the first to sixth optical network units (ONUs) 160a to 160n be fixed so as to be prevented from being moved in position.
A tunable optical transceiver (T-TRX) used in either the optical line terminal (OLT) 100 or the first to sixth optical network units (ONUs) 160a to 160n has a tendency in which an output optical wavelength is easily changed by a factor such as a temperature change of external air. In the case where the tunable optical transceiver (T-TRX) is deviated from a predetermined wavelength position by an external factor such as a temperature change of external air, communication is interrupted or an interference occurs between the optical wavelength and a wavelength of another channel, resulting in a serious deterioration of communication quality.
Therefore, there is a need for development of a technology that can implement wavelength locking or wavelength stabilization of a light source used in the wavelength division multiplexing passive optical network (WDM-PON) in a simpler and more effective manner.
To this end, an invention of Korean Patent application No. 10-2007-0088904 entitled “Wavelength Tunable Passive Optical Subscriber Network and Wavelength Control Method thereof” has been proposed. However, such a proposed Korean Patent entails a problem in that a locking time is long, communication may not be carried out due to incorrect wavelength, and a communication connection is established while both the OLT and ONU perform a slow optical scanning operation, leading to an increase in the communication channel setting time.